The remote locations, exposed surface areas and extensive layouts of solar power plants put them at high risk of damage from the elements, particularly electrical storms. A significant concern for photovoltaic (PV) power plant operators is equipment damage caused by direct or indirect lightning strikes. Damage from these events can bring a PV installation offline for days or perhaps weeks, resulting in power interruption and revenue losses. To avoid the destructive effects of lightning strikes, overvoltage protection must be installed at the inverter and at various other locations in the PV facility.
Scelta e installazione dei dispositivi di protezione contro le sovratensioni per impianti fotovoltaici
Gli impianti fotovoltaici hanno caratteristiche uniche, che richiedono quindi l'utilizzo di SPD specificatamente progettati per gli impianti fotovoltaici.
I sistemi fotovoltaici hanno tensioni di sistema CC elevate fino a 1500 volt. Il loro punto di potenza massima funziona solo a pochi percentili al di sotto della corrente di cortocircuito del sistema.
Per determinare il modulo SPD corretto per il sistema fotovoltaico e la sua installazione, è necessario conoscere:
– the lightning round flash density;
– the system’s operating temperature;
– the system’s voltage;
– the system’s short circuit current rating;
– the level of waveform that is to be protected against (indirect or direct lightning); and
– the nominal discharge current
Per avere un effetto protettivo, il livello di protezione della tensione di un SPD (Up) dovrebbe essere inferiore di 20 % rispetto alla rigidità dielettrica dell'apparecchiatura terminale del sistema.
È importante utilizzare un SPD con una corrente di cortocircuito superiore alla corrente di cortocircuito della stringa del campo solare a cui è collegato l'SPD. L'SPD fornito sull'uscita CC deve avere un MCOV CC pari o superiore alla tensione massima del sistema fotovoltaico del pannello.
When a lightning strikes at point A (see Figure 1), the solar PV panel and the inverter are likely to be damaged. Only the inverter will be damaged if the lightning strikes at point B. However, the inverter is typically the most expensive component within a PV system, which is why it is essential to properly select and install the correct SPD on both the ac and dc lines. The closer the strike is to the inverter, the more damaged the inverter will be.
DC & AC Side SPDs for combiner boxes and inverters
Surge-protection devices are installed on the DC and AC sides of central and string inverters, in combiner boxes and to protect signal lines (measurements and communications). Depending on the atmospheric exposure of the PV plant to lightning, engineering, procurement and construction (EPC) contractors and designers may prescribe either standard type 2 IEC SPDs (indirect impact 8/20 μs) or more robust type 1+2 SPDs (tested to withstand direct lightning impacts in 10/350 μs waveform).
DC side protection at combiner boxes and inverters typically has a Y-connection DIN-rail configuration in order to provide common and differential mode protection.
On the AC side, overvoltage protection of the inverter is likewise required. Whether at the inverter itself or at the AC combiner box, exposure to voltage surges is generally addressed by using standard DIN-rail type 2 SPDs in TN S, IT or even TT configurations.
Moreover, depending on the PV installation, type 1+2 AC may be favored in some cases by EPC contractors and designers, in order to achieve a longer lifetime for SPDs and even protection where conditions are harsher in terms of the expected direct lightning impact. This will typically be the case in areas with a very high isoceraunic level, often associated with high altitudes.
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